1. Field of the Invention
The present invention relates to an optical clock recovery apparatus and method useful in, for example, the optical repeaters of a long-haul fiber-optic communication system.
2. Description of the Related Art
Optical communication networks are transporting increasing quantities of data over increasing distances. With the increasing distances come increased problems of optical signal degradation, including waveform degradation in both the temporal and frequency domains. The causes are optical loss, signal-to-noise-ratio degradation owing to the use of multiple optical amplifier stages, and waveform distortion owing to group velocity dispersion. As transmission distances increase, these problems become increasingly apparent.
For these reasons, repeaters are placed at intervals of a few tens of kilometers to a few hundred kilometers along the transmission route to regenerate optical signals by restoring their frequency waveforms and temporal waveforms to their original form.
To regenerate an optical signal, it is necessary to recover a clock signal synchronized with the clock signal that was used to generate the optical signal. The clock signal may be a train of pulses spaced on the time axis at intervals such that the pulse repetition rate is equal to the bit rate of the optical signal. Alternatively, the clock signal may be a sinewave signal with a frequency equal to the bit rate. In either case, the clock signal must be recovered from a degraded version of the optical signal.
The clock signal may be recovered as either an electrical signal or an optical signal. The term ‘clock signal’ will be used herein to refer to an electrical clock signal. If the clock signal is an optical signal, it will be referred to as an optical clock signal.
The input optical signal from which the clock signal or optical clock signal is recovered may have been generated from an optical clock signal of the pulse train type by modulating the pulse train according to a binary data signal. If an electrical clock signal is recovered, it may be used to recover data from the input optical signal, and then used to drive a semiconductor laser to produce an optical clock signal, which is modulated by the recovered data to regenerate the input optical signal.
One known method of recovering a clock signal from an input optical signal with a known bit rate is to use a photodiode, for example, to convert the input optical signal to an electrical signal and filter the electrical signal with a narrow-band electrical filter having a passband centered on or near the bit rate frequency, thereby selecting the frequency component of the electrical signal that matches the bit rate. The selected component is an electrical pulse train or sinewave with a repetition rate or frequency equal to the bit rate, that is, a recovered clock signal.
To meet the increasing demands placed on optical communication networks, much study has focused on methods of transmitting multiple channels in a multiplexed manner, and in particular on optical time division multiplexing (OTDM). In OTDM transmission, the bit rate of the multiplexed signal increases in direct proportion to the number of multiplexed channels. When the bit rate of an OTDM signal becomes as high as forty gigabits per second (40 Gb/s), at the present state of the art, recovery of an electrical clock signal becomes difficult, because the electronic devices used as optical-to-electric conversion elements and as components of electrical filters lack the operating speed necessary to keep up with the bit rates of multiplexed optical transmission.
Accordingly, all-optical clock recovery methods are being studied. These methods bypass optical-to-electrical conversion, and can recover an optical clock signal directly from an OTDM signal having a bit rate too high for the operating speeds of current electronic devices. The basic strategy is to feed the input optical signal into a mode-locked laser that generates optical pulses with a repetition rate approximating the bit rate of the input optical signal. Descriptions of this technique can be found in Japanese Patent Application Publications No. 07-506231 (now Japanese Patent No. 3510247 to Smith) and No. 09-167870 (now Japanese Patent No. 2751903 to Yano), and in the following publications:
Ono et al., ‘Optical clock extraction from 10-Gbit/s data pulses by using monolithic mode-locked laser diodes’, OFC'95 Technical Digest, ThL4.
Ludwig et al., ‘40 Gbit/s demultiplexing experiment with 10 GHz all-optical clock recovery using a modelocked semiconductor laser’, Electronics Letters, Vol. 32, No. 4, pp. 327-329, 1996
Bao et al., ‘Impact of Saturable Absorption on Performance of Optical Clock Recovery Using a Mode-Locked Multisection Semiconductor Laser’ IEEE Journal of Quantum Electronics, Vol. 40, No 9, pp. 1177-1185, 2004
In a first method of all-optical clock recovery, described by Smith, a mode-locked laser of the fiber type is used. The optical pulses generated by lasing action are modulated by cross phase modulation (XPM) with the input optical signal, due to the optical Kerr effect, as they travel back and forth in the optical fiber. This causes the laser pulses to synchronize with the optical pulses in the input optical signal, so that when they exit the laser they form a recovered optical clock signal.
In a second method of all-optical clock recovery, described by Yano, Ono et al., Ludwig et al., and Bao et al., a passive mode-locked semiconductor laser employing a saturable absorber is used. The optical absorption coefficient of the saturable absorber changes in synchronization with the optical pulses of the input optical signal. As a result, the pulses of laser light generated in the optical resonator cavity of the passive mode-locked semiconductor laser synchronize with the optical pulses in the input optical signal and become a recovered optical clock signal. Recovery of a 160-Gb/s optical clock signal by use of a passive mode-locked semiconductor laser that generates an optical pulse train with a repetition frequency of 160 GHz has been reported by Arahira (the present inventor) and Ogawa in ‘Retiming and Reshaping Function of All-Optical Clock Extraction at 160 Gb/s in Monolithic Mode-Locked Laser Diode’, IEEE Journal of Quantum Electronics, Vol. 41, No. 7, pp. 937-944, 2005, indicating that methods using a passive mode-locked semiconductor laser are useful for all-optical recovery of clock signals from ultra-high-speed optical pulse signals, such as OTDM signals.
In a third method of all-optical clock recovery (described by Brox et al. in ‘Self-Pulsating DFB for 40 GHz Clock-Recovery: Impact of Intensity Fluctuations on Jitter’, OFC 2004 Technical Digest MF55), the optical signal is input to a self-pulsating laser, causing it to generate optical pulses at a repetition frequency synchronized to the bit rate frequency of the input optical signal. Brox et al. use a distributed feedback (DFB) laser with multiple electrodes, which allows the repetition frequency of the optical pulses to be varied over a wide range by varying the driving conditions of the electrodes.
A first major problem with the three methods described above is that in addition to a mode-locked laser or a self-pulsating laser they require a wavelength filter with a passband positioned at the lasing wavelength of the laser to remove the wavelength component of the input optical signal from the optical output of the mode-locked laser or self-pulsating laser, as noted by Bao et al. in the second paragraph in the left column of page 3 of the cited reference, and by Brox et al. in the second column of page 2 of the cited reference.
The problems that arise if the wavelength component of the input optical signal is not removed from the optical output of the laser will be explained in the detailed description of the present invention, with reference to experimental results. The need for an optical wavelength filter can also be readily inferred from the Yano patent. Although no wavelength filter is shown in FIG. 2 of this patent document, the input light injected into the laser propagates in the same direction as the light output by the lasing action of the laser, so the output laser light must be separated from the spent input light. In FIG. 3A in the patent to Smith, a wavelength division multiplexing (WDM) filter is inserted for the apparent purpose of preventing unwanted wavelength components of the input optical signal from being included in the recovered optical clock signal.
The need for an optical wavelength filter increases the size, number of components, and cost of an all-optical clock recovery device employing any of the above methods. A more basic problem is that if the input optical signal has the same or nearly the same wavelength as the recovered clock signal, then no wavelength filter can exclude unwanted input optical signal components from the recovered optical clock signal. With a mode-locked laser, a further problem is that if the input optical signal and the recovered optical clock signal have the same wavelength, or nearly the same wavelength, the operation of the mode-locked laser becomes unstable due to optical injection locking, making it impossible to recover the optical clock signal.
A second major problem with the three methods above is that the optical clock signal recovery apparatus will not operate properly unless special care is taken. If a mode-locked laser is used, it must be driven so as to maintain a fixed relationship between the wavelength of the input optical signal and the wavelength of the recovered optical clock signal in order to control optical injection locking, as explained in paragraphs 0028 to 0037 of the Yano publication. Effective control of optical injection locking enables the frequency and phase of the lasing mode of the mode-locked laser to be controlled, but maintaining the stability of this state is extremely difficult with present control techniques.
When the wavelength of the input optical signal is so close to the wavelength of the recovered optical clock signal as to be inseparable by a wavelength filter, the mode-locked laser can easily slip between the optical injection locked state, in which the wavelength and phase of the mode-locked semiconductor laser are pulled toward and locked at the wavelength and phase of the input optical signal, and the non-locked state in which there is no fixed phase relationship between the input optical signal and the laser light. As a result, random optical beats arise between the lasing spectrum of the mode-locked semiconductor laser and the wavelength components of the input optical signal, making recovery of a stable optical clock signal difficult.
If optical injection locking could be controlled, it could be used to recover an optical clock signal, but there would still be practical problems, because the lasing spectrum of the mode-locked semiconductor laser would differ considerably, depending on whether the input optical signal was present or absent, as shown by Yokoyama et al. in FIG. 6 in the article ‘Synchronous Injection Locking Operation of Monolithic Mode-Locked Diode Lasers’, Optical Review Vol. 2, No. 2, pp. 85-88, 1995. In this figure the lasing spectrum of the mode-locked semiconductor laser is concentrated around 1590 nanometers (nm) when the input optical signal is absent, but widens by being pulled down to the vicinity of 1585 nm when an input optical signal is present.
In the example described by Yokoyama et al. the input optical signal is an optical pulse train, so the conditions do not completely match the conditions under which an optical clock signal is normally recovered, but the mode-locked laser operates by the same principle, so the same change in the lasing spectrum of the mode-locked semiconductor laser can be expected to occur in optical clock recovery from actual input optical signals.
It follows that to recover an optical clock signal by use of optical injection locking, it would be necessary to take the changes in the lasing spectrum of the mode-locked semiconductor laser due to the input optical signal into account in the design of the mode-locked semiconductor laser. The changes in the spectrum of the optical clock signal recovered by a mode-locked semiconductor laser also depend on the wavelength of the input optical signal, and this too would have to be considered in the design of the mode-locked semiconductor laser.
A further problem with the use of optical injection locking is that if the input optical signal had a chirped frequency characteristic, the chirp would be transferred to the laser light output by the mode-locked semiconductor laser, that is, the recovered optical clock signal. The wavelength spectrum of the recovered optical clock signal would then include unwanted variations acquired from the chirped frequency characteristic of the input optical signal.
When optical injection locking occurs, at least one of the longitudinal lasing mode components of the mode-locked semiconductor laser matches the wavelength of the input optical signal. That is, there is a fixed correlation between the wavelength of the input optical signal and the wavelength of the recovered optical clock signal. More precisely, the wavelengths of the light output by the longitudinal lasing modes of the mode-locked semiconductor laser are limited to a group of wavelengths separated from the wavelength of the input optical signal by integer multiples of a wavelength equivalent to the repetition rate of the optical pulse train output from the mode-locked semiconductor laser.
The wavelengths of the longitudinal lasing modes of the mode-locked semiconductor laser are therefore not independent of the wavelength of the input optical signal. In the design of a practical optical clock signal recovery apparatus, the wavelength of the input optical signal and the wavelength of the recovered optical clock signal cannot be set arbitrarily. This constrains the design of practical optical clock signal recovery apparatus.
This constraint poses particular problems in systems in which the use of wavelength division multiplexing is contemplated. A specific problem is that optical injection locking may occur between the input optical signal and a longitudinal lasing mode of the mode-locked semiconductor laser that differs from the longitudinal lasing mode with maximum spectral intensity. In this case, if the wavelength of the input optical signal matches a particular grid wavelength, the center wavelength of the recovered optical clock signal, that is, the wavelength of the longitudinal mode with the maximum spectral intensity, will not in general match this particular grid wavelength, or any other wavelength in the WDM grid. That is, the wavelength of the recovered optical clock signal will belong to a wavelength grid different from the wavelength grid used in the WDM system, making the optical clock signal unusable in the WDM communication system without further alteration.
Even if optical clock signal recovery is carried out with the intensity of the input optical signal reduced to a level at which optical injection locking does not occur, the spectral characteristics of the recovered optical clock signal will still vary depending on the wavelength of the input optical signal. Further details will be given later, but the reason for this problem is that even without optical injection locking, a multiple modulation effect occurs due to optical resonance effects in the mode-locked semiconductor laser, and as a result, an effective modulation intensifying effect occurs. This problem becomes pronounced when the wavelength of the input optical signal approaches the lasing wavelength of the mode-locked semiconductor laser, making it susceptible to optical resonance effects. When this happens, a slight change in the wavelength of the input optical signal can produce large changes in the characteristics of the recovered optical clock signal, including its temporal jitter, pulse width, lasing spectrum, and so on. These problems must be worked around in the design of a practical optical clock signal recovery apparatus using a mode-locked semiconductor laser.
In consideration of optical injection locking and optical resonance effects, the relationship between the lasing spectrum of a mode-locked semiconductor laser and the wavelength of the input optical signal requires close attention in the design of a practical optical clock signal recovery apparatus. Besides adding complications to the design of the optical clock signal recovery apparatus, these considerations can restrict its range of use. An example is the restriction arising from the wavelength grid in a WDM system as described above.
The major problems of the conventional art are accordingly the difficulty of recovering an optical clock signal that does not include components of the input optical signal, and the tendency of the wavelength spectrum of the recovered optical clock signal to change when the wavelength of the input optical signal changes. Because of these problems, with the known types of optical clock signal recovery apparatus it is difficult to recover an optical clock signal that does not include components of the input optical signal and has the desired clock frequency, that is, has a repetition rate that matches the bit rate of the input optical signal, despite changes in the wavelength of the input optical signal.